The present invention relates to signal tracking and, more particularly, to an apparatus and method for tracking satellite signals.
A Global Navigation Satellite System (GNSS) tracks a position of a target on the ground using the network of artificial satellites flying in space. The GNSS refers to a Global Positioning System (GPS) operated by United States Department of Defense, a Galileo Positioning System being developed by European Union, and a GLObal NAvigation Satellite System (GLONASS) operated by Russia. The GNSS is extensively employed to acquire the location of aircrafts, vehicles, ships, etc. and also adapted for information technologies such as telematics and the like.
Typically, the GPS is a satellite navigation system for providing position information using satellites which revolve around the orbit of space. The GPS was initially constructed for military purposes and has been widely used in various fields after Selective Availability (SA) was removed since the year 2000. GPSs adopt a Code Division Multiple Access (CDMA) method and share a band using different codes based on the same frequency band.
The CDMA-based GPS uses a direct sequence spread spectrum by which the frequency band of a signal can be spread much wider than that of an original signal. A GPS receiver needs to accurately synchronize a received GPS signal and a pseudo noise (PN) code in order to reproduce desired information from the received GPS signal.
The GPS receiver includes a correlator for down-converting a signal received through an GPS antenna to a baseband signal and dispreading the signal. The signal received through the GPS antenna are generally converted into an intermediate frequency (IF) signal and are then quantized on a 2-bit basis. The correlator is configured to down-convert the IF signal to the baseband signal and to track the IF signal using CDMA codes such as Coarse/Acquisition (C/A) codes.
The correlator generally generates three signals having a ½ chip interval therebetween in order to track a satellite signal. The correlator generates a Prompt signal, an ½ Early signal which is a ½ chip earlier than the Prompt signal, and a ½ Late signal which is a ½ chip later than the Prompt signal. Conventionally, the GPS receiver filters the satellite signal every 2 MHz and rarely has a gain although the chip interval is reduced to less than ½ chip in the baseband processing process.
Recently, an RF module having a bandwidth of 4 MHz or more has been designed. If the correlator generates only signals having a ½ chip interval in the bandwidth of 4 MHz or more, it may be difficult to overcome error resulting from multi-paths. Accordingly, the correlator having a ¼ chip interval has emerged. In more detail, a Prompt signal, a ¼ Early signal which is a ¼ chip earlier than the Prompt signal, a ½ Early signal which is a ½ chip earlier than the Prompt signal, a ¼ Late signal which is a ¼ chip later than the Prompt signal, and a ½ Late signal which is a ½ chip later than the Prompt signal are used.
FIG. 1 shows graphs showing the relationships between correlation values resulting from a direct path and correlation values resulting from multi-paths. A graph (a) of FIG. 1 shows correlation values using ideal ½ chip delay resulting from a direct path. The correlation value of a ¼ chip Late (or Early) signal is ¾ times the correlation value of a Prompt signal. Further, the correlation value of a ½ chip Late (or Early) signal is ½ times the correlation value of the Prompt signal. Tracking makes correlation values of the ¼ chip Late (or Early) signal and ½ chip Late (or Early) signal to maintain same ratio. An error due to the multi-paths is determined by using the correlation values of the ¼ chip Late (or Early) signal and ½ chip Late (or Early) signal and then is compensated. A graph (b) of FIG. 1 shows correlation values when multi-path signals having the same phase exist. A graph (c) of FIG. 1 shows correlation values when multi-path signals having a phase shift of 180° exist. From graphs (a), (b) and (c), it can be shown that the correlation values under multi-path are non-symmetric although correlation values under direct path are symmetric. Nevertheless, the correlation values of the ½ chip Late (or Early) signal of multi-path as shown in graph (b) is symmetric. This means that wrong tracking may be performed when only the ½ chip Late (or Early) signal is used. Accordingly, by using the correlation values of the ½ chip Late (or Early) signal as well as the correlation values of the ½ chip Late (or Early) signal, more accurate tracking can be performed.
The complexity of the correlator may increase as the chip interval becomes narrow. For example, in order to support the ¼ chip interval, nine integration circuits may be necessary as compared with the case where the ½ chip interval uses five integration circuits. Accordingly, problems may arise because the design of the correlator becomes complicated and the capacity of memory required is increased.